It is well known that the properties of thermoplastic polymers and thermoplastic/filler composite material can be enhanced by orienting the thermoplastic molecules and filler (when present). Further, for certain applications it is desirable to orient the molecules and filler in two directions. Accordingly much research has been directed towards developing methods for producing such products.
One method proposed for producing biaxially oriented products is solid state extrusion. For example U.S. Pat. No. 4,801,419 (Ward et al., 1989) and Ward et al., Plast. Rub. Comp. Proc. Appl. 19 (1993) describe a process to produce uniaxially and biaxially oriented hollow profiles by deforming a pre-extruded, unoriented hollow workpiece comprising orientable, thermoplastic polymer by passage in the solid phase through a die having both an entry and an exit side. The hollow workpiece is provided at the entry side of the die, while tension is applied to the hollow workpiece from the exit side of the die. Two important elements disclosed in these publications are the temperature of the polymer during deformation and the drawing or tension applied to cause this deformation. The temperature must be well below the melting point of the polymer, thus, the deformation of polymer took place in the solid phase. The tension applied must be high enough to cause solid state deformation but low enough so as not to cause tensile failure. A special grip is needed to hold and pull the workpiece from the exit side of the die. As the result, this process is a batch process.
A similar process is described in U.S. Pat. No. 5,096,634 (Tsadares and Anastassakis, 1992) which describes a process for producing biaxially stretched, unfilled thermoplastic tube (especially polyvinylchloride) by steps of extruding an unoriented thermoplastic tube through a die in the melt/viscous state, cooling the tube and drawing it at the same speed as the extrusion speed, passing the tube next over a conical expanding portion of a stretching mandrel (a second die) and drawing the expanded tubing at the exit at a speed higher than the entering speed, thus, biaxially stretch the tube. The stretching is resulted from the differences in the drawing speed of the tubing before and after the conical die.
It is desired to produce hollow profiles or sheet with increased mechanical strength properties in two directions, where the strength properties in both directions can be designed or engineered according to the desired needs. It is also desirable to produce these superior products continuously using commercially available equipment at a cost comparable to or lower than existing products.